Light cycle oil (hereinafter also referred to as LCO), which is a cracked gas oil produced in a fluid catalytic cracking, contains a large amount of polycyclic aromatic hydrocarbons, and has been used as a gas oil or a heating oil. However, in recent years, investigations have been conducted into the possibilities of obtaining, from LCO, monocyclic aromatic hydrocarbons of 6 to 8 carbon number (such as benzene, toluene, xylene and ethylbenzene), which can be used as high-octane gasoline base stocks or petrochemical feedstocks, and offer significant added value.
For example, Patent Documents 1 to 3 propose methods that use zeolite catalysts to produce monocyclic aromatic hydrocarbons from the polycyclic aromatic hydrocarbons contained in large amounts within LCO and the like.
However, in the production of monocyclic aromatic hydrocarbons of 6 to 8 carbon number using the catalysts disclosed in Patent Documents 1 to 3, the yield of monocyclic aromatic hydrocarbons of 6 to 8 carbon number during the initial stages of reaction were not entirely satisfactory. Further, the steady-state yield of monocyclic aromatic hydrocarbons of 6 to 8 carbon number has also tended to be low.
When monocyclic aromatic hydrocarbons are produced from a heavy feedstock oil containing polycyclic aromatic hydrocarbons, large amounts of carbon matter are deposited on the catalyst, causing a rapid deterioration in the catalytic activity, and therefore a catalyst regeneration process that removes this carbon matter must be performed frequently. Further, in those cases where a circulating fluidized bed is employed, which is a process in which the reaction and catalyst regeneration are repeated in an efficient manner, the temperature for the catalyst regeneration must be set to a higher temperature than the reaction temperature, resulting in a particularly severe temperature environment for the catalyst.
Under these types of severe conditions, if a zeolite catalyst is used as the catalyst, then the catalyst tends to suffer from hydrothermal degradation, causing a reduction in the steady-state yield of the monocyclic aromatic hydrocarbons, and therefore improvements in the hydrothermal stability of the catalyst are required. However, the zeolite catalysts disclosed in Patent Documents 1 to 3 employ no measures to improve the hydrothermal stability, and offer very little practical usability.
Examples of known methods for improving the hydrothermal stability include a method that uses a zeolite having a high Si/Al ratio, a method in which the catalyst is subjected to a preliminary hydrothermal treatment to stabilize the catalyst, such as USY zeolite, a method in which phosphorus is added to a zeolite, a method in which a rare earth metal is added to a zeolite, and a method that involves improving the structure-directing agent used during the synthesis of a zeolite.
Of these methods, the addition of phosphorus not only improves the hydrothermal stability, but also provides other known effects such as an improvement in selectivity due to suppression of carbon matter deposition during fluid catalytic cracking, and an improvement in the abrasion resistance of the binder. Accordingly, this method is frequently applied to catalysts used in catalytic cracking reactions.
Examples of catalytic cracking catalysts prepared by adding phosphorus to a zeolite include those disclosed in Patent Documents 4 to 6.
Namely, Patent Document 4 discloses a method for producing olefins from naphtha using a catalyst containing ZSM-5 to which has been added phosphorus, as well as gallium, germanium and/or tin. In Patent Document 4, phosphorus is added for the purposes of suppressing the production of methane and aromatics in order to enhance the selectivity for olefin production, and ensuring a high degree of activity even for a short contact time, thereby improving the yield of olefins.
Patent Document 5 discloses a method for producing olefins in a high yield from heavy hydrocarbons by using a catalyst prepared by supporting phosphorus on ZSM-5 containing zirconium and a rare earth element, and a catalyst containing a USY zeolite, an REY zeolite, kaolin, silica and alumina.
Patent Document 6 discloses a method for producing ethylene and propylene in a high yield by transforming hydrocarbons using a catalyst containing ZSM-5 having phosphorus and a transition metal element support thereon.
As mentioned above, the addition of phosphorus to zeolites has been disclosed in Patent Documents 4 to 6, but in each of these documents, the main purpose was improvement of the olefin yield, and monocyclic aromatic hydrocarbons of 6 to 8 carbon number were not able to be produced at high yield. For example, Table 2 in Patent Document 6 discloses the yields for olefins (ethylene and propylene) and BTX (benzene, toluene and xylene), and whereas the yield for the olefins was 40% by mass, the yield for BTX was a low value of approximately 6% by mass.
Accordingly, a catalyst for producing monocyclic aromatic hydrocarbons that is capable of producing monocyclic aromatic hydrocarbons of 6 to 8 carbon number in a high yield from a feedstock oil containing polycyclic aromatic hydrocarbons, not only during the initial reaction but also under steady-state conditions, is not currently known.